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p. 109) are characteristic of the group, and the latter bone is
peculiar to the Monotremata among mammals. So, too, is the
large coracoid. In the scapula there is a spine which coincides
with the anterior border of that bone. The arrangement of the
muscles in this region proves conclusively that this projection is
the homologue of the spine and the acromion of other mammals.
Here, again, we have a point of likeness to the Cetacea.1 In
the pelvis the acetabulum is perforate (in JScJiidna), as in

Considering the numerous very
archaic features which the general
structure of this group displays,
it is surprising to find how typi-
cally mammalian they are in
certain other peculiarities. The
mammalian diaphragm, one of
the distinguishing features of the
class, is perfectly normal in the
Monotremata. The alimentary
canal shows no great divergences
from the normal structure. The

Stomach   is  almost   globular, With   FIG. 52,—Side view of right half of the

a projecting pyloric region in
Ornithorhync7ius; the intestine
is divided into a " small" and
" large" intestine by a slender
caecum. The liver has the sub-
divisions that this organ usually
shows in the Mammalia. How-
ever, the presence of the ventral mesentery and of the
abdominal vein in JBchidna and OrnitJiorhynchus has already
been mentioned as a distinctive character. The peculiar and
apparently partly primitive valve of the right ventricle has been
described above (see p. 66). The brain is in most respects
mammalian in its characters, but naturally shows some important
differences. Dr. Elliot Smith, who has most recently studied
this question,2 is of opinion that the size of the cerebral hemi-
spheres is not at all reptilian ; indeed, it " greatly exceeds that of

1 Muscular insertions and attachments do not, however, altogether support the
comparison.                                          8 Joum* Anat. fhys. 1899, p. 309.

shoulder girdle of a young Echidna
(JSchidna aculeata}. x 1. «, Acro-
mion ; c, coracoid; c&, coracoid border;
cl, clavicle ; ess, coraco-scapular
suture ; ec, epicoracoid ; gbt glenoid
border ; gc, glenoid cavity ; i&, inter-
clavicle j jj?/i postscapular fossa ; jos*
presternuin ; s, spine ; ss, suprascapii-
lar epiphysis ; ssf, subseapular fossa.
(From Flower's Osteology.)